It is highly desirable in obstetrics to provide information concerning the condition of the fetus both before and during labor. Fetal monitors have been developed for use in monitoring and recording both fetal heart rate and uterine activity.
Routine monitoring in labor currently relies on charting the fetal heart rate (FHR) and uterine contractions on a chart paper to produce a cardiotocogram (CTG). Measurement of these two variables can be made by way of either external or internal monitoring. In the case of external monitoring, Doppler ultrasound is used to detect fetal heart movements from which the heart rate can be derived, whilst an external tocodynamometer is used for the detection of uterine wall tension. However, the FHR and uterine activity measurement may be inaccurate and therefore in cases of high risk patients or suspected fetal compromise, internal monitoring is preferred.
In conventional internal monitoring techniques, a fetal scalp clip is attached to the skin of the fetal scalp. The fetal ECG is obtained from the scalp and detection of the QRS complexes enables measurement of the heart rate. An intrauterine pressure catheter for the direct measurement of intrauterine pressure (IUP) is also generally employed. This procedure necessitates the insertion of two devices, and mothers generally object to the use of a scalp clip piercing the skin of the baby. Furthermore, the scalp clip allows vertical transmission of viral infections from the mother to the baby (e.g. HIV or hepatitis B viruses).
More recent work in the field of internal monitoring has led to the development of a flexible intrauterine probe that lies alongside the fetus in utero. The probe is inserted transcervically in one single clinical procedure. It is fitted with surface electrodes which are in contact with the fetal head, along with an intrauterine pressure sensor. Such a probe is described in U.S. Pat. No. 5,025,787, a significant advantage of such a device being that it is non-invasive to the fetus (unlike both ultrasound and scalp clips) and requires minimal clinical procedure. The contents of U.S. Pat. No. 5,025,787 are included herein by reference.
The signal obtained by an intrauterine probe contains both the fetal and maternal ECG complexes. Such composite signals are incompatible with conventional cardiotocographs and it is therefore necessary to separate the maternal and fetal complexes to obtain an FHR recording. In the case of signals recorded by way of electrodes placed on the maternal abdomen, it is usually the case that the maternal complexes have an amplitude considerably greater than the fetal complexes, and this fact can be used for separation. In the case of signals recorded with an intrauterine probe no such assumption can be made. In some cases the maternal complexes are larger than the fetal ones, in others they are smaller or similar in size. Additionally, the polarity of both fetal and maternal complexes varies from one labor to the next, and sometimes during the course of a labor. It is to be noted that the abdominal ECG monitoring method referred to above suffers from the disadvantage that it usually cannot be used during labor, as the electromyographic signal from the uterine contractions often obscures the fetal ECG.
Techniques for deriving the FHR from a composite signal have been developed which rely on obtaining the maternal EGG from another source (e.g. maternal chest derived signals) and using this information to eliminate the maternal complexes by subtraction or by masking. However, such an approach suffers from the need for additional leads and electrodes placed on the maternal abdomen. These can be uncomfortable to the mother and can interfere with other clinical procedures during labor, especially if the monitoring lasts several hours or more. Furthermore, separate monitoring of the maternal ECG affords more scope for measurement errors to arise.
German patent application DE-A-2717530 discloses a proposed method of suppressing unwanted signal components in abdominally derived feto-maternal ECGs. This method involves preliminary filtering of the input signal, followed by the use of the cross correlation function between the filtered signal and a stored sample "interference signal". The interference signal identified is then subtracted from the composite signal. It is important to note that the method described depends on the maternal signal exhibiting a greater amplitude than the fetal signal, and that therefore signal amplitude threshold detectors can be used to identify the maternal QRS complexes (as explained above, such relative signal characteristics cannot be assumed in the case of signals derived from an intrauterine source). Once the maternal signal has been removed, the correlation analysis can be conducted to identify regularly occurring interference elements.
U.S. Pat. No. 5,123,420 describes a method for processing signals indicative of two or more heart rates (e.g. of two fetuses or of one fetus and its mother). The method relies on the detection of an FHR by means of a fetal scalp electrode or an ultrasound transducer, as well as the detection of a further fetal and/or a maternal heart rate by appropriate means. According to the technique, cross-correlation is used to compare two traces and provide a warning when coincidence is detected, thus warning that both detecting means are inadvertently recording the maternal trace and that consequently the fetal heart is not being monitored.
U.S. Pat. No. 5,025,787 makes reference to the fact that the maternal and fetal ECG complexes have different widths and generally different morphologies, and that this fact might be used to distinguish between them. It is suggested that frequency domain pattern recognition of the spectral components, or temporal/spatial pattern recognition, can be applied. However, the document does not teach a method which succeeds in providing effective discrimination between the signal components.
There therefore continues to exist a need to monitor the FHR by way of, for example, an intrauterine probe, which takes into account the maternal ECG contamination of the measured signal, without the need to additionally obtain the maternal ECG from another source. The end result for the clinical user should be a reliable and unambiguous FHR trace.